![]() ![]() This, however, fails to be predicted by the previous model without incoherent radiations. Moreover in such a system, the ground-state coherence contributed by incoherent environment is found to give rise to the considerable enhancement of excitation energy transfer. Our general effective theory is verified in the pigment-protein complex by uncovering the long-lived electronic and excitonic coherences caused by vibrational coherence. Our motivation is to understand the role of vibrational coherence on the long-lived coherence and also the efficient energy transfer measured in recent experiments 14, 15, 16. Our general theory, on the other hand, suggests a physical mechanism for slowing down the dynamical decoherence, which is potentially applicable in quantum computation. This leads to much longer survival of quantum coherence than that with bare excitons only. Intuitively this configuration gives rise to the screening of the exciton-bath interaction, in analogy to the Yukawa potential between electrons in the interacting electron gas 27. ![]() As a result, the new composite called polaron emerges, resulting in the suppression of system-reservoir coupling. The bare electron is surrounded by discrete vibrational modes whose dynamics will be taken into account, due to their strong coupling and subsequently comparable relaxation timescale with electrons. ![]() In this article we develop an effective theory to explore in a general scenario the underlying mechanism for long-lived quantum coherence, while other investigations numerically show the existence of such effect without illustrating the mechanism at rigorous level 18, 24. The persistence of coherences originating from the vibrational modes has been evidently observed in recent experiment 26. Therefore the exciton-vibraiton interaction have to be explicitly considered in this case. This is in contrast with the non-adiabatic case where phonons cannot be easily averaged out due to their slow relaxation. Such effect is not considered in the adiabatic regime, since the phonons can be averaged out because of the much faster relaxation of phonons than that of excitons. Here the non-adiabaticity explicitly refers to the case including the phonon dynamics owing to the comparable relaxation of phonons with the excitons. Thus this phonon dynamics often has a crucial effect on the energy transport when the energy quanta of vibrational modes are in resonance with the energy splitting of excitons 22, 23, 24, 25. In fact, some discrete intramolecular vibrations are of comparable time scale with the relaxation of exciton, which subsequently leads to the breakdown of adiabatic approximation 18, 19, 20, 21. Although the models under this approximation were somehow successful in describing the population dynamics of excitons in a quasi-classical way, they show their failure on explaining the long-lived coherence oscillations 7, 12 observed in 2D femtosecond electronic spectroscopy 13, 14, 15, 16, 17. ![]() Even with the knowledge of electronic structure in antenna and the advances in spectroscopy that uncovered the long-lived quantum coherence in noisy environment 5, 6, the full understanding of the role of coherence and mechanism of excitation energy transfer has still remained mysterious.Ĭonventionally, either in Förster theory or the advanced models including dephasing 7, 8, 9, 10, the excitonic energy transfer is considered in adiabatic framework under Born-Oppenheimer approximation 11 where only the degrees of freedoms of the electrons are included. The transport of excitation energy in the antenna is remarkably fast and efficient, usually with quantum yields close to 100% 4. Recently the widespread interest in exploring the quantum nature in solar cells and the photosynthetic process has been triggered by experimental investigations of excitonic dynamics in light-harvesting and Fenna-Matthews-Olson (FMO) complexes 1, 2, 3. This is attributed to the nonequilibriumness of the system caused by the detailed-balance-breaking, which funnels the downhill migration of excitons. The ground-state vibrational coherence generated by incoherent radiations is shown to be long-survived and is demonstrated to be significant in promoting the excitation energy transfer. To test the validity of our theory, we study the pigment-protein complex in detail by exploring the energy transfer and coherence dynamics. The estimation of the timescale of coherence elongated by vibrational modes is given in an analytical manner. This subsequently demonstrates the role of vibrational coherence which greatly contributes to long-lived feature of the excitonic coherence that has been observed in femtosecond experiments. We explore the mechanism for the long-lived quantum coherence by considering the discrete phonon modes: these vibrational modes effectively weaken the exciton-environment interaction, due to the new composite (polaron) formed by excitons and vibrons. ![]()
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